Pope Benedict XVI greets a pilgrim representing her country during his inaugural Mass in St Peter's Square on Sunday. (Reuters)

Vatican City, April 24 (Reuters): Pope Benedict XVI took charge of his Church at a majestic inauguration today and set down a spiritual manifesto for his papacy with a plea for humanity to escape a desert of suffering and embrace God.

Three weeks after the death of John Paul, Presidents and pilgrims again packed the cobbled expanse in front of St Peter's Basilica to see the new Pontiff installed on the papal throne as the leader of 1.1 billion Roman Catholics.

Cloaked in shimmering golden vestments, Benedict told a 350,000-strong crowd that he was 'a weak servant of God' and appealed for prayers to help him in the 'enormous task that truly exceeds human capacity'.

He again promised to continue John Paul's policy of reaching out to Jews and other faiths and pays his first visit outside the Vatican tomorrow to St Paul Outside the Walls, a Rome basilica with a special place in the quest for Christian unity.

The thrust of his homily, however, was a call for the world to rediscover its spirituality and a plea for support in a task he said 'truly exceeds all human capacity'.

In his first sermon as the 265th leader of the Church, delivered entirely in Italian, he focused on what he called a world of alienation, suffering and death that he said had become a spiritual wasteland.

Facing such woes, he said his Church was still very much alive, young and able to grow. 'There are so many kinds of desert. There is the desert of poverty, the desert of hunger and thirst, the desert of abandonment, of loneliness, of destroyed love,' the Pope said.

'There is the desert of God's darkness, the emptiness of souls no longer aware of their dignity or the goal of human life. The external deserts in the world are growing, because the internal deserts have become so vast.

The Pontiff, formerly Cardinal Joseph Ratzinger, was the austere guardian of Catholic orthodoxy under John Paul, but today he looked to soften his image and deflect concerns over his past as a tough doctrinal enforcer.

'My real programme of governance is not to do my own will, not to pursue my own ideas, but to listen, together with the whole Church, to the word and the will of the Lord,' said the 78-year-old German to loud applause.

Applause echoed around the colonnaded square as flag-waving pilgrims interrupted his powerful sermon more than 40 times, chanting 'Benedict, Benedict,' at the end of the speech.

'I liked his homily a lot. He took up the previous Pope's words. This should be a papacy of continuity,' said Silvio Viccierhai, a 50-year old Italian.

At the end of the two-and-a-half hour Mass, the Pope was driven around the square in a white, open-topped vehicle, smiling and waving as he passed through a sea of cheering pilgrims.

Benedict, the oldest man to be elected Pope for three centuries, takes over the Church at a time of dwindling congregations and an ageing base in Europe and stiff competition from evangelical sects for followers in the developing world.